The Glomeromycota is the newest fungal phylum. These are the arbuscular mycorrhizal (AM) fungi that form symbioses with the majority of land plant species. Fossil and molecular evidence suggest this is an ancient symbiosis, that may have been instrumental in enabling plants to colonise terrestrial habitats. The AM fungi gain carbon from their plant host, and the primary benefit to plants is thought to be the acquisition of phosphate, a highly immobile ion in soil. However, they are thought to have many other ecological benefits. Host specificity has been considered to be low in this group, as most fungi in culture will colonise most plants, but recent evidence suggests that some AM fungi show a degree of specificity. We review the evidence revealing the genetic structure of this ancient group of fungi. Although a consensus has not yet been reached, we suggest that understanding the evolution and genetic structure of this enigmatic group may be the key to understanding how they function in ecosystems.

Responses of Medicago truncatula to colonization by two arbuscular mycorrhizal fungi, Scutellospora calospora isolate WUM 12(2) and Glomus caledonium isolate RIS 42, were compared in the light of previous findings that the former fungus can be ineffective as a beneficial microsymbiont with some host plants. The plants were grown individually in two-compartment systems in which a lateral side arm containing soil labelled with 33P was separated from the main soil compartment by a nylon mesh that prevented penetration by roots but not fungal hyphae. Fungal inoculum was applied as a root–soil mixture in a band opposite the side arm. Nonmycorrhizal controls were set up similarly, without inoculum. There were harvests at 28, 35, 42 and 49 d. Both sets of mycorrhizal plants grew better than nonmycorrhizal plants and initially had higher concentrations of P in shoots and roots. Plants grown with S. calospora grew better than plants grown with G. caledonium, and this was associated with somewhat greater fungal colonization in terms of intraradical hyphae and numbers of arbuscules. Scutellospora calospora formed denser hyphae at root surfaces than G. caledonium. By 28 d there were extensive hyphae of both fungi in the side arms, and after 35 d S. calospora produced denser hyphae there than G. caledonium. Nevertheless, there was very little transfer of 33P via S. calospora to the plant at 28 d, and thereafter its transfer increased at a rate only c. 33% of that via G. caledonium. The results showed that plants colonized by S. calospora preferentially obtained P from sites in the main soil chamber relatively close to the roots, compared with plants colonized by G. caledonium. Hence formation of a highly beneficial arbuscular mycorrhizal symbiosis does not necessarily depend on development of hyphae at a distance from the roots or on large-scale translocation of P from distant sites. The results are discussed in relation to previous studies with compartmented systems that have involved the same fungi. Possible causes of the variable effects of S. calospora in symbiosis with different host plants are briefly assessed. Differences in spatial abilities of individual arbuscular mycorrhizal fungi to acquire P might have strong ecological implications for plant growth in soils low in P.

Arbuscular mycorrhizas are a widespread symbiosis between soil fungi and plant roots. Flow cytometry, after DNase I partial digestion and DAPI staining, and light and electron microscopy were used to analyse chromatin condensation and nuclear conditions in mycorrhizal and control roots of Allium porrum. The 2C peak, detected by flow cytometry, split into two peaks representing two populations of nuclei, one more resistant and one more susceptible to the enzyme action. The microscopic analyses showed the presence of pyknotic and chromatolytic nuclei, two typical features of senescence. In order to quantify the senescing process, a terminal deoxynucleotidyl transferase assay was performed on extracted nuclei, later analysed by flow cytometry. The numbers of senescing nuclei and their DNA cleavage were higher in control plants. Our results show the existence of senescing nuclei in cortical cells of the bulbous monocotyledon A. porrum and the delaying effect of arbuscular mycorrhizas on senescence.

The involvement of the cytoskeleton in symbiotic interactions such as arbuscular mycorrhizas has received little attention. In this paper, we examine the organization of actin in tobacco mycorrhizal roots and compare actin and tubulin patterns within arbuscule-containing cells.

Our results show drastic reorganization of microfilaments and microtubules upon fungal infection and how those new cytoskeletal patterns relate to the host cytoplasm rearrangement and the intracellular fungal structures. Whereas in uninfected cells a network of cortical and perinuclear actin filaments was observed, in infected cells actin filaments closely follow the fungal branches and envelop the whole arbuscule in a dense coating network. Microtubules are less closely connected with the fungus surface. They run across the whole arbuscule mass, linking branches to each other and to the host cell cortex and nucleus.

These major differences between the two cytoskeletal components are used to advance some suggestions concerning their contribution to structural functions in the plant–fungus interactions during the mycorrhizal symbiosis.

To analyse the effect of arbuscular mycorrhizal (AM) colonization on tomato gene expression, two-dimensional polyacrylamide gel electrophoresis (2D-PAGE) patterns of crude extracts, soluble and membrane proteins of tomato roots, either mycorrhizal and the AM fungus Glomus mosseae (Nicol. & Gerd.) Gerd. & Trappe or non-mycorrhizal, have been compared. In the three fractions analysed, AM colonization induced up-regulation with down-regulation of the synthesis of polypeptides already present in tomato roots and induction of some new polypeptides. Separation of root extracts into soluble and membrane fractions allowed us to identify two soluble, and five membrane-bound, newly induced polypeptides in AM roots. Comparison of the protein patterns of AM roots with those of the external mycelium of G. mosseae showed that one of the newly induced polypeptides might correspond to a fungal polypeptide. By using this experimental approach, we have been able to detect 44 polypeptides that are differentially displayed in tomato roots as a consequence of the establishment of the AM symbiosis.

The frequent co-occurrence of ectomycorrhizas and arbuscular mycorrhizas in the same ecosystem raises the question about how two functionally different mycorrhizal associations exploit a spatially heterogeneous habitat. Horizontal (in plots along a transect of 3750 m crossing forest types of low and high abundance of ectomycorrhizal (ECM) host species) and vertical (across profiles of litter and 0–5 cm, 10–15 cm, 25–35 cm soil layers) distribution of ectomycorrhizas and arbuscular mycorrhizas in relation to edaphic parameters (pH, moisture, total carbon, total nitrogen and extractable-phosphorus concentration) was compared in Korup National Park rain forest, to test the hypothesis that the two types of mycorrhiza exploit different niches. No pattern was observed in either ECM or arbuscular mycorrhizal (AM) fractional colonization with depth in the profile even though most of the variation in soil parameters was vertical. Only local (between monoliths within plots) horizontal differences were found in soil parameters. By contrast, horizontal differences in fractional colonization were found between forest types, and between monoliths, for ectomycorrhizas but only between monoliths for arbuscular mycorrhizas. The lack of a clear relationship between both horizontal and vertical distribution of ECM and AM fractional colonization and soil parameters suggests that both fungal groups are equally able to colonize the same niche, at the scale of our experiment. By contrast, a negative relationship was found between AM and ECM fractional colonization in the litter and 0–5 cm and 10–15 cm soil layers. This, in addition to the more heterogeneous horizontal distribution of arbuscular mycorrhizas in forest areas dominated by ectomycorrhizas and a patchy vertical distribution of the former as opposed to a more coherent distribution of the latter, suggests that both types of mycorrhizal fungi compete for the same niche. The co-occurrence of ectomycorrhizas and arbuscular mycorrhizas might be explained by differences between ECM and AM hosts and fungi in their colonizing abilities.

The present work describes the morphogenesis and cytological characteristics of ‘branched absorbing structures’ (BAS, formely named arbuscule-like structures, ALS), small groups of dichotomous hyphae formed by the extraradical mycelium of arbuscular mycorrhizal (AM) fungi. Monoxenic cultures of the AM fungus Glomus intraradices Smith & Schenck and tomato (Lycopersicum esculentum Mill.) roots allowed the continuous, non-destructive study of BAS development. These structures were not observed in axenic cultures of the fungus under different nutritional conditions or in unsuccessful (asymbiotic) monoxenic cultures. However, extraradical mycelium of G. intraradices formed BAS immediately after fungal penetration of the host root and establishment of the symbiosis. The average BAS development time was 7 d under our culture conditions, after which they degenerated, becoming empty septate structures. Certain BAS were closely associated with spore formation, appearing at the spore's substending hypha. Branches of these spore-associated BAS (spore-BAS) usually formed spores. Electron microscopy studies revealed that BAS and arbuscules show several ultrastructural similarities. The possible role of BAS in nutrient uptake by the mycorrhizal plant is discussed.

Arbuscular mycorrhizal (AM) fungi, Rhizobium bacteria and plant-growth-promoting rhizobacteria (PGPR) were isolated from a representative area of a desertified semi-arid ecosystem in the south-east of Spain. Microbial isolates were characterized and screened for effectiveness by a single-inoculation trial in soil microcosms. Anthyllis cytisoides L., a mycotrophic pioneer legume, dominant in the target mediterranean ecosystem, was the test plant. Several microbial cultures from existing collections were also included in the screening process. Two AM fungi (Glomus coronatum, native, and Glomus intraradices, exotic), two Rhizobium bacteria (NR4 and NR9, both native) and two PGPR (A2, native, and E, exotic) were selected. A further screening for the appropriate double and triple combinations of microbial inoculants was then performed. The parameters evaluated were biomass accumulation and allocation, N and P uptake, N2-fixation (15N) and specific root length. Overall, G. coronatum, native in the field site was more effective than the exotic G. intraradices in co-inoculation treatments. In general, our results support the importance of physiological and genetic adaptation of microbes to the whole environment, thus local isolates must be involved. Many microbial combinations were effective in improving either plant development, nutrient uptake, N2-fixation or root system quality. Selective and specific functional compatibility relationships in plant response between the microbial inoculants, were observed. Despite the difficulty of selecting a multifunctional microbial inoculum, appropriate microbial combinations can be recommended for a given biotechnological input related to improvement of plant performance. This could be exploited in nursery production of target plant species endowed with optimized rhizosphere/mycorrhizosphere systems that can be tailored to help plants to establish and survive in nutrient-deficient, degraded habitats. The relevance of this microbial-based approach in the context of a reclamation strategy addressed to environmental sustainability purposes is discussed.

We compared the plasma membrane, tonoplast and mitochondrial Mg-ATPase activities of microsomes isolated from mycorrhizal and non-mycorrhizal sunflower (Helianthus annuus L.) and onion (Allium cepa L.) roots during a time-course experiment (21–60 d). No significant difference was observed between the protein levels of root microsomes from mycorrhizal and non-mycorrhizal sunflower. However, the protein content of microsomes obtained from mycorrhizal onion roots increased when the symbiosis was well established (more than 50% of root length colonized by the arbuscular mycorrhizal fungus). Kinetic studies revealed that vanadate-sensitive (plasma membrane-associated) ATPase activity of sunflower root microsomes increased with mycorrhizal colonization. However, the vanadate-sensitive ATPase activity of microsomes from mycorrhizal onion roots decreased compared with those from non-mycorrhizal controls. Increased activity of nitrate (tonoplast-associated) and azide (mitochondrial membrane-related)-sensitive ATPase activities was detected in extracts obtained from 45- and 60-d-old mycorrhizal onion plants. A slight increase in azide-sensitive ATPase activity was detected in microsomes of mycorrhizal sunflower. The existence of different physiological and biochemical strategies in sunflower and onion in relation to the establishment of this mutualistic symbiosis is discussed.

Spring ephemerals have evolved specific growth strategies that take advantage of the high photon fluence rates of early spring. These strategies involve a sequential growth of different organs. Erythronium americanum (Ker-Gawl), a common spring ephemeral of northern maple forests, produces its roots in the autumn, although stems and leaves develop in the spring. Mycorrhizal infection of the root system occurs very rapidly and intensively. The fungi thus depend on carbohydrate reserves accumulated in the corm (underground reserve organ) for their growth and development through the winter. We found that the presence of mycorrhizas drastically decreases root growth during the cold period and is more costly of carbohydrate reserves than root growth alone. However, during the spring, the presence of mycorrhizas fully benefits the plant: we measured an annual growth rate of mycorrhizal plants twice that of fungicide-treated plants which were non-mycorrhizal. We suspect that E. americanum is highly susceptible to water stress during the growing season and might rely on its mycorrhizas for water supply to a greater extent than for nutrient supply. Nutrient uptake might occur mainly in the autumn when arbuscules are at their most abundant.